The practice of medicine and the technology surrounding it continue to evolve in a dramatic fashion. One manifestation of this evolution is the increase in the average human life span which alone spawns the need for still further development, particularly in the area of implantable medical articles, such as in the area of prostheses which either replace or support failing, diseased or deteriorated anatomical parts.
Synthetic materials are known to be useful in the manufacture of many of these implantable articles. Among the more useful synthetic materials in this application are fibers formed from synthetic polymers which are substantially non-resorbable and resistant to degradation in the body. These are often highly desirable for many implantable article applications because of their mechanical properties such as tensile strength, flexibility, and elasticity. Furthermore, their ability to be engineered into useful structures and retain these mechanical properties under the conditions present in the human body can be desirable. For example, certain synthetic fibers formed from polyester have been used in the manufacture of vascular grafts, exhibiting sufficient strength to withstand the pressure of arterial or venous flow while also exhibiting flexibility and recovery. Additionally, fabric vascular grafts possess the versatility to be conveniently implanted into the body without losing structural strength.
For many applications of synthetic polymer fibers in the body, however, the fibers that retain suitable mechanical properties (such as polyethylene terephthalate and polypropylene) do not provide the desired biologic response. Articles formed of these synthetic fibers often have the risk of a negative reaction, for example, chronic inflammation, thrombosis and intimal hyperplasia, sometimes with potentially fatal results.
Fibers made from polymers that can be resorbed by the body (often called "resorbable polymers") can provide a positive and desired biologic response in the body. Examples of such resorbable polymers are polyglycolic acid and polylactic acid and copolymers of glycolic with lactic acid or .epsilon.-caprolactone or trimethylene carbonate. In addition to these polyesters are the polyester-ethers such as poly-p-dioxanone.
However, because these polymers are resorbable, the fibers made from them and implanted in the human body typically lose their mechanical properties in a time period considerably shorter than the needed life of an implant. These materials are more typically used as a scaffold for the growth and organization of implanted organ cells. Parenchymal cells are isolated from the desired tissues and seeded into the polymer, and the cell-polymer structure is implanted. While the scaffold gradually deteriorates, the implanted cells proliferate and secrete substances that form an extracellular matrix (ECM). The growing cells, ECM, and vascular tissue continually replace the void spaces of the disappearing scaffold until eventually the implant has been replaced by natural tissue. A spectacular example of this approach is an artificial--but living--human external ear formed on a polymer matrix implanted subcutaneously on the back of a laboratory animal. However, this approach may not be suitable for replacement of tissues that are subject to continuous physical/mechanical demands, such as heart and blood vessel walls, fasciae, ligaments and tendons, bursae and other joint tissues. Therefore, these fibers alone may not be optimal for use in implant applications.
Thus a major problem is inherently present in implantable prosthetic articles that are formed from materials such as synthetic fibers because of the prior art fibers' inability to both retain good long term mechanical properties and also elicit a positive and desirable biologic response. One particular area in which this combination of desired properties has not been achieved is in the use of small bore (.about.&lt;6 mm diameter) vascular grafts. Availability of suitable small diameter vascular grafts could significantly expand the opportunities for vascular repair since arteries having diameters in this range, such as the radial artery and the arteries in the Circle of Willis, provide a major component of the blood supply to key organs and extremities and are often in need of repair due to injury or disease.
It is also known from Poiseuille's Law that flow through blood vessels is proportional to the fourth power of the vessel radius. Reducing the diameter of a blood vessel by half, as may occur in intimal hyperplasia or partial thrombosis, reduces the vessel's blood flow to 1/16th the original flow. Due to the small size and, in general, low blood flow, these small grafts put an even greater demand on maintaining a clear cross-sectional area for blood flow. Unfortunately, there are currently no synthetic vascular grafts that work well in this application. This is a severe problem and one for which a truly workable fiber and vascular graft fabric would be a major step forward.
Prior attempts to provide fibers with both long term mechanical properties and biocompatibility have been largely unsuccessful. One approach has been to use resorbable fibers and biologically stable fibers such as those described above together in an implantable fabric such as a vascular graft. This approach is described, for example, in EPO Application 0 202 444 (Nov. 26, 1986) and U.S. Pat. No. 4,997,440 (Mar. 5, 1991).
However, published results have shown that these types of constructions fail because even a modest amount (20%) of the non-resorbable polymer fibers exposed and present inside the body (in this case polyethylene terephthalate) significantly inhibited the desired biologic response (J. Vasc. Surgery, 3 (5), May 1986). Furthermore, experimental results indicate that even at nominal levels of the non-resorbable component, problems of fabric failure and/or defects leading to potential aneurysm still exist as indicated in U.S. Pat. No. 4,997,440 which indicates from "slight" to "significant" aneurysmal tendency in grafts with 25% to 33% non-resorbable fiber.
Another approach to address these limitations has been to coat the prosthetic fabric. For example, EPO Application 334 046 discloses a surgical composite which is manufactured by extruding a non-resorbable polymer into a fiber, fabricating the fiber into a textile structure and then encapsulating the structure with a resorbable polymer. Such an approach has several drawbacks including changing the relative flexibility of the basic fabric graft. Additionally, by virtue of the coating process, the resulting fabric is less likely to have the small open pores and interstitial channels which are useful for cell and tissue ingrowth from outside the graft through the wall into the vessel lumen. Further, such coating processes do not reliably provide a desirable uniform coating layer on individual filaments with controlled thickness of the coating layer.
A continuing need therefore still exists for a practical and economically manufacturable, useable implantable prosthetic article which exhibits the properties necessary to perform the desired function for the prosthetic within the body while maintaining its structural and functional integrity and performance and bringing about a positive biologic response leading to healing and the desired functioning of the prosthesis.